Developer for bichromated colloids



Patented Aug. 14, 1951 Paul J. Whyzmuzis, .iainaica, and William 0.

Kaupp, @zone Park, N. Y., assignors to Enterchemical Corporation, New York, N. TL, a corporation of @hio No Drawing. Application May 14, 1949, Seriai No. 93,431

(ill. 95-88) 3 @laims.

This invention relates to a lithographic process, and is aimed particularly at the production of an improved developing composition for the typical light-sensitive bichromate gum films which are used as resists in the process. Specifically, this invention is directed at compositions particularly adapted for processing bimetallic and deep etch plates.

In the production of deep-etch and bi-metallic printing plates for offset lithography, a metal plate is first coated with a solution of a water dispersible colloid (gum arabic, other water soluble gumscarboxy methyl cellulose, etc.) which contains a bichromate to make it light sensitive. This coating is exposed, through a screened positive, to intense light, for a sufficient time to harden the colloid in the non-printing areas. The exposed plate is then developed-e. g. the non-hardened portions are dissolved-and the plate is then finished by etching the exposed metal, coating the etched metal with lacquer and developing ink, and finally stripping the colloid. The developer must dissolve the unexposed portion without attacking the light-hardened portion of the film.

The sharpness of the dots is a function of the exposure time, and the developing speed. All normal photographic dots have a fringe about them; the fringe will protect the colloid from the light for a time, but can eventually be pierced. Hence, longer exposure gives sharper dots. Furthermore, the developer can be used to control sharpnessthe less the development, the sharper the dot. However, there is little leeway in ordinary development, since all the dots must be developed if true reproduction is to be ob= tained.

In processing bi-metallic plates, which generally carry relatively fine grains as compared with conventional deep etch plates, the plates carry thinner films of coating because of the finer grain size. This means that for identical exposures, the dots produced are sharper for the conventional plates--the thinner film gives less opportunity for undercutting by the light used. The difference in exposure necessary is not greatonly a minute or two-but on a step and repeat machine this may cause trouble. Hence, it is desirable to produce the desirable sharpening of the dots with the developer. Attempts to do this with conventional developers have always resulted in lack of detail.

We have discovered how to produce a satisfactory slow developer which does not result in loss of detail. The key ingredient in our developer is hydroxy-acetic acid;

Our preferred slow developer for bi-metallic plates consists of a combination, in aqueous solution, of calcium chloride, zinc chloride, and hydroxy-acetic acid. It can be made as follows:

3.49 pounds calcium chloride-tech. 2.40 pounds zinc chloride 5.02 pounds water 2 are heated together to solution, omitting a portion of the water, cooled to '70 F., and adjusted to 43.5-43.7 Baum with the withheld water. Ordinarily, total water is 5.02--but control is necessary in case the anhydrous chemicals have picked up water.

This solution is then mixed as follows:

62.0 pounds base solution 1.0 79% aeueous acid (or approximately 1.1% of hydroxy-acetic acid).

The speed of the developer can be controlled by the amount of hydroxy-acetic acid addeda superior developer for ordinary deep etch plates can be made by increasing the content of hydroxy acetic acid to a maximum of 2% pounds of acid per 62 pounds of base solution (about 2.9% hydroXy-acetic acid).

If the acid content is reduced below 1% fine detail is lost. Thus, the effective range is from about 1 t0 3%.

If the base solution is diluted, the amount of acid necessary is reduced; but as the solution is diluted, the chance of dissolving the light hardened portions of the coating is increased. More concentrated solutions are difficult to keep in solution, and are slower in action.

The calcium chloride zinc chloride ratio cannot be increased above the 3 to 2 ratio in these concentrated solutions, or it may precipitate; however, the calcium chloride zinc chloride weight ratio can be reduced to about 1 to 1 without causing undue loss of control.

We claim:

1. A developer for bichromated water soluble colloid coatings comprising a concentrated aqueous solution of calcium chloride and Zinc chloride in a weight ratio between 3 to 2 and l to 1, and from 1 to 3% by weight of hydroxy-acetic acid.

2. A developer for bichromated water-soluble colloid coatings comprising by weight an aqueous solution of about 32% calcium chloride, 22% zinc chloride and from 1 to 3% hydroXy-acetic acid.

3. A slow developer for bichromated water soluble colloid coatings comprising by weight an aqueous solution of about 32% calcium chloride, about 22% zinc chloride and about 1.1% hydroxy-acetic acid.

PAUL J. WHYZMUZIS. WILLIAM O. KAUPP.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,785,635 Lebedenko Dec. 16, 1930 2,265,829 Wood Dec. 9, 1941 FOREIGN PATENTS Number Country Date 413,428 Great Britain July 19, 1934 

1. A DEVELOPER FOR BICHROMATED WATER SOLUBLE COLLOID COATINGS COMPRISING A CONCENTRATED AQUEOUS SOLUTION OF CALCIUM CHLORIDE AND ZINC CHLORIDE IN A WEIGHT RATIO BETWEEN 3 TO 2 AND 1 TO 1, AND FROM 1 TO 3% BY WEIGHT OF HYDROXY-ACETIC ACID. 